1. Field of the Invention
The present invention relates to a kneading apparatus for resin compounding, and a method for producing a toner for developing an electrostatic image, used in an electrophotographic image forming apparatus.
2. Description of the Related Art
The term “resin compounding” generally refers to kneading and dispersion of functional fillers for the purpose of imparting functions to resins serving as bases. Examples of the imparted functions include conductivity, charging property, magnetism, thermal conductivity, piezoelectricity, vibration suppressing property, sound insulating property, sliding property, heat insulating property, lightness, light scattering/reflecting property, heat ray radiating property, flame-retardant property, radiation protection, ultraviolet protection, dehydrating property, color generating property and releasing property.
Examples of the functional fillers include carbon black, graphite, ferrite, magnetic iron oxide, alumina, barium titanate, lead zirconate titanate, mica, potassium titanate, xonotlite, carbon fibers, lead powder, barium sulfate, molybdenum sulfide, Teflon (registered trademark) powder, talc, glass balloons, Shirasu balloons, charcoal powder, titanium oxide, glass beads, calcium carbonate, aluminum powder, magnesium oxide, hydrotalcite, dawsonite, zinc oxide, iron oxide, calcium oxide, magnesium oxide, pigments and waxes.
Specific examples of applied products of the resin compounding include an electrostatic image developing toner obtained by dispersing a pigment, a wax, a charge controlling agent, etc. in a resin, a pigment masterbatch obtained by dispersing a pigment in a resin, a flame-retardant plastic obtained by dispersing a flame retardant in a resin, and a foaming agent masterbatch obtained by dispersing a foaming agent in a resin (refer to p. 337, “Base technology and High technology applications of Mixing & Dispersion”, published by Techno System).
Resin compounding methods for kneading and dispersing functional fillers are broadly classified into the batch kneading method and the continuous kneading method. The batch kneading method is problematic in that the kneading temperature is difficult to control and so the quality easily varies from batch to batch, and also problematic in that long-time operation is required, which leads to low throughput and low productivity. Due to such problems with the batch kneading method, the continuous kneading method is becoming popular as a present-day resin compounding method.
The most typical kneading apparatus for use in the continuous kneading method (hereinafter referred to also as “continuous kneading apparatus”) would be the biaxial-screw continuous kneading apparatus in which shear force generated between two screws that are disposed parallel and close to each other is applied to a heated and melted resin so as to knead and disperse functional fillers into the base resin.
However, regarding the present-day resin compounding, there is an increasing need for fine dispersion of fillers to improve functionalities further. In order for a biaxial-screw continuous kneading apparatus to meet this need, it is necessary to extend the apparatus in an axis direction to thereby increase an effective kneading area. This is because the effective kneading area is an area where the two screws are close to each other and thus, in order to meet the need for fine dispersion of the fillers, it is necessary to extend, as the kneading area, the area where the two screws are close to each other. When the apparatus is extended in an axis direction, however, there are problems such as an increase in the size of the apparatus and an increase in costs.
To meet the need for fine dispersion of the fillers in the present-day resin compounding, note is taken of stone mortar type continuous kneading apparatuses (refer to Japanese Patent Application Publication (JP-B) Nos. 02-000092 and 54-024743 and Japanese Patent Application Laid-Open (JP-A) No. 52-148868). The following explains a stone mortar type continuous kneading apparatus.
A stone mortar type continuous kneading apparatus includes a cylindrical stationary portion provided with an internal space through which a heated and melted resin can pass, and also includes a rotary portion which is placed in the internal space of the stationary portion and rotates thereby continuously kneading the resin passing through the internal space and, while doing so, conveying the resin in the rotational axis direction. The stationary portion is provided with an annular stationary disc placed such that the diameter of the internal space partially decreases, and the rotary portion is provided with a drive shaft member to which drive is transmitted from a drive source, and a rotary disc member fixed to the drive shaft member with its disc center penetrated by the drive shaft member. The rotary disc member is placed such that its circular surface faces the annular surface of the stationary disc, the surface of the rotary disc member and the surface of the stationary disc facing each other are provided with concave portions and convex portions shaped like mountains and valleys, and the rotary disc member and the stationary disc constitute a kneading area in the form of a stone mortar. By the rotary disc member rotating with respect to the stationary disc, the resin present in the gap between the rotary disc member and the stationary disc is subjected to shearing and thus kneaded and dispersed, while being moved, as in the case of a stone mortar. In such a stone mortar type continuous kneading apparatus, the kneading area is formed in a direction perpendicular to the rotational axis direction, so that it can perform kneading more efficiently than the biaxial-screw continuous kneading apparatus in which the area where the two screws are close to each other functions as the kneading area. Hence, the stone mortar type continuous kneading apparatus does not necessitate increasing the length thereof with respect to the axis direction in meeting the need for fine dispersion of the fillers, and thus fine dispersion of the fillers can be realized with a compact, low-priced apparatus in comparison with the biaxial-screw continuous kneading apparatus.
Nowadays, there is a need for finer dispersion of the fillers, and even the stone mortar type continuous kneading apparatuses cannot sufficiently meet the need in some cases.
In a stone mortar type continuous kneading apparatus, shear force acts on a resin present in the gap between a rotary disc member and a stationary disc; when the temperature of the resin present in the gap is too high, the viscosity of the heated and melted resin decreases, thereby making it difficult for the shear force to act effectively and thus making finer dispersion of the fillers difficult.
In the continuous kneading apparatus described in JP-B No. 02-000092, a stationary portion is provided with a cooling medium passage through which a cooling medium that is lower in temperature than a resin to be kneaded passes, but a rotary portion is not provided with a cooling medium passage, and thus it is not possible to cool the resin to a temperature suitable for shear force to act effectively in a kneading area.
Meanwhile, in each of the continuous kneading apparatuses described in JP-A No. 52-148868 and JP-B No. 54-024743, cooling passages are provided for a drive shaft member and a rotary disc member fixed to the drive shaft member, with the drive shaft member and the rotary disc member constituting a rotary portion. Thus, it is possible to make the temperature of a resin to closer to a temperature suitable for shear force to act in a kneading area than in the case of the continuous kneading apparatus described in JP-B No. 02-000092. However, it has turned out that the structure in which a cooling medium is passed through the drive shaft member and the rotary disc member yields poor resin cooling efficiency. The following is the reason for this.
Regarding the rotary portion in each of the continuous kneading apparatuses described in JP-A No. 52-148868 and JP-B No. 54-024743, it is desirable, for maintenance purposes, that a member which comes into contact with the resin to be kneaded be produced as a member different from the drive shaft member and that the member which comes into contact with the resin be fixed to the drive shaft member. Specifically, there is desirably a structure in which a screw member that comes into contact with the resin on the upstream side of the rotary disc member with respect to the conveyance direction of the resin and that rotates, thereby providing the resin with conveyance force advancing in the rotational axis direction, and the rotary disc member are produced as members different from the drive shaft member, and the screw member and the rotary disc member are fixed to the drive shaft member.
Coming into contact with the resin, the screw member and the rotary disc member could be temporally abraded or chipped owing to a temporary load, and so they need to have replaceable structures. Also, unless the screw member and the rotary disc member are separable from the drive shaft member, the entire rotary portion needs replacing when abrasion or chipping has arisen, thereby leading to an increase in running costs. In the case where the screw member and the rotary disc member are members different from the drive shaft member and are separable from the drive shaft member, only an abraded or chipped member can be replaced when abrasion or chipping has arisen, thereby making it possible to reduce running costs. Furthermore, replacement with a screw member having a different shape and/or a rotary disc member having a different shape makes it easily possible to alter conveyance conditions and/or kneading conditions to some extent and thus to yield a structure suitable for maintenance.
In the case where the screw member and the drive shaft member are different members, the heat transmission efficiency between the screw member and the drive shaft member is poor, and the resin positioned in contact with the screw member is poorly cooled even when a cooling medium is passed inside the drive shaft member.
In the kneading area, the resin increases in temperature by frictional heat generated when the shear force acts, and the increase in temperature can be suppressed by passing a cooling medium inside the rotary disc member; note that cooling can be performed more efficiently by a cooling medium performing a cooling function before the temperature increase in the kneading area. However, as described above, a cooling function does not easily work in the position in contact with the screw member, where the resin passes before increasing in temperature in the kneading area; therefore, the structure in which the cooling medium is passed through the drive shaft member and the rotary disc member yields poor resin cooling efficiency.
When the resin cooling efficiency is poor, the temperature of the resin to be supplied into the kneading area cannot be sufficiently lowered, shear force cannot be adequately applied to the resin, and thus finer dispersion of the fillers is impossible to achieve.
Also, in a process of producing an electrophotographic toner, if fine dispersion of fillers into a resin serving as a base is insufficient in a kneading step in which toner materials are melted and mixed together using a continuous kneading apparatus, it may be impossible to exhibit the functions required for the toner at the time of image formation, which could lead to a decrease in image quality.
The present invention is primarily aimed at solving the problems in related art and achieving the following objects.
A first object of the present invention is to provide a continuous kneading apparatus capable of efficiently cooling matter to be kneaded, and applying adequate shear force to the matter.
A second object of the present invention is to provide a method for producing a toner, capable of obtaining a toner wherein filler(s) is/are dispersed sufficiently finely in resin(s) serving as a base.